In a process for manufacturing semiconductor devices or the like, a plasma process using a plasma is frequently performed. As shown in FIG. 9, a plasma processing apparatus for the plasma process includes, for example, a process vessel 10 of a vacuum chamber, and a stage 11 disposed in the process vessel 10 for supporting thereon a semiconductor wafer (hereafter referred to as “wafer”) as a substrate. The stage 11 also serves as a lower electrode. A showerhead 12 having a number of gas supply holes 12a is arranged above the stage 11. An upper electrode 13 is disposed on a lower surface of the showerhead 12. The upper electrode 13 is formed of an electrode plate 13a and a cooling block 13b. 
A radio-frequency for generating plasma is applied between the upper electrode 13 and the lower electrode 11 (stage 11) from a radio-frequency power source 15. Thus, a plasma is generated in a process space between the stage 11 and the upper electrode 13. The plasma activates a process gas supplied from the showerhead 12 into the process vessel 10, and then the wafer W on the stage 11 is subjected to a plasma process such as an etching process and a film-deposition process.
A structure and properties of the upper electrode 13 in the process vessel have an impact on the plasma process. Specifically, in a plasma etching process, for example, an in-plane uniformity or inter-plane uniformity in etching rate of the wafer(s) W is affected by the structure and properties of the upper electrode 13. A structure for adjusting a temperature, which is also one of such impacting factors, is required to make uniform temperature in an upper surface of the same diameter region of the wafer W. A known structure for the upper electrode 13 satisfying this requirement is that the electrode plate 13a made of ceramics or a conductive material, free of heavy metal contamination, is placed at a position in contact with a process area, and that the cooling block 13b is laid on the electrode plate 13a. The cooling block 13b is positioned in a vacuum gas atmosphere, so that the cooling block 13b has to be provided with an intricate coolant channel meandering through the gas supply holes 12a so as not to interfere with gas supply.
There is used the cooling block 13b formed by joining two metal plates by brazing. A channel 14 through which a cooling liquid flows is formed in the cooling block 13b. The cooling block 13b should have both an excellent thermal conductivity so as to fulfill a desired heat exchange function, and an excellent electric conductivity as a passage for a radio-frequency. Thus, a metal plate made of stainless steel (hereafter referred to as “SUS”) having a high resistance and a high thermal conductivity cannot be used. In place thereof, there is generally used a metal plate made of aluminum (Al) whose resistance and thermal conductivity are much lower than those of SUS.
However, when aluminum is used, since an aluminum solid surface is exposed to the cooling liquid channel 14 which is in contact with a cooling liquid, there is a possibility that an inner peripheral surface of the channel 14 is corroded by the cooling liquid circulating therethrough. The corrosion of the inner peripheral surface of the channel 14 may then result in a blockage of the cooling liquid in the channel 14. Thus, in order to avoid such situation, the inner peripheral surface of the channel 14 formed in the cooling block 13b must be subjected to an anti-corrosion treatment.
One of the anti-corrosion treatment methods is to form an aluminum oxide film on the inner peripheral surface of the channel 14 by alumite coating. However, in joining the metal plates whose surfaces have been subjected to an alumite coating process before brazing, there is concern that an alumite coating film is cracked if the alumite coating film cannot resist a brazing temperature. The cracked alumite coating film cannot fully achieve an anti-corrosion function. Alternatively, the metal plates can be subjected to the alumite coating process after the metal plates have been joined to each other by brazing. In this case, an electrolytic solution for alumite coating is poured into the channel 14. However, the complicated structure of the channel 14 may inhibit introduction of the electrolytic solution through the channel 14. Then, parts which are not coated with alumite may be left in the channel 14, i.e., pin holes may be generated in the channel 14. Further, there is concern that a foreign matter is deposited in the channel 14 by a reaction of dissolved oxygen and alumite, causing a blockage in the channel 14.
Another anti-corrosion treatment method is to coat the inner peripheral surface of the channel 14 with a resin. However, also in this method, since it is necessary to pour the resin into the channel 14, the complicated structure of the channel 14 may inhibit the pouring of the resin. Then, parts which are not coated with alumite may be left in the channel 14, i.e., pin holes may be generated in the channel 14. In addition, it is uncertain whether such cooling block 13b provides a sufficient thermal conductivity.
Another method is to arrange a pipe in the cooling block 13b. However, it is significantly difficult to intricately bend a pipe. Further, the pipe will not withstand a brazing temperature (about 600° C.) at a brazing step. Thus, this method cannot be adopted.
If a chiller is used, a convenient material such as aluminum and copper can be used, because this method is free from the need for considering the anti-corrosion property. However, it is often the case that the method is obliged to be abandoned, in terms of costs and spaces.)
JP2002-86295A (especially sections 0002 and 0019 discloses a method of manufacturing a composite used in a radiator in an automobile. That is to say, there is provided a composite used as a flat tube. The composite includes three laminates, i.e., a wax member containing Si, an aluminum alloy core member, and a sacrificial member made of an Al—Zn based alloy, which are combined to each other by electric-resistance welding, with the wax member in the composite facing outside. The composite is manufactured by applying a sacrificial agent to one surface of the core member, and hot-rolling the laminates. However, there is no suggestion for a suitable structure and manufacturing method of a cooling block which forms an electrode for generating plasma for use in a plasma process.